1. Field Invention
This invention relates to an apparatus and method for varying the inductance of an electrical circuit by changing the spacing of the inductance coil windings.
2. Description of Related Art
Devices for varying electrical inductance are known in the art. Frequently such devices are used to tune antenna circuits to obtain resonance at a desired frequency. Typically an antenna is designed for a specific frequency, by sizing the active members of the antenna, and is effective for a range of frequencies usually centered on that specific resonant frequency. Some antenna designs have multiple resonant frequencies, and some are relatively effective over a very broad range of frequencies. Others, due to size limitations, are short relative to the desired wavelength and require an inductive load to offset the capacitive load provided by the shortness of the antenna compared to the desired frequency. Increasing or decreasing the inductance on the antenna with a loading device, assuming the physical length of the antenna is electrically short, decreases or increases the resonant frequency respectively. Several devices for varying the inductance of an antenna have been taught.
A common device to vary inductance is to have an inductor of fixed length which has the effective length adjusted by a contact, or contacts, that move along the length of the inductor coil, varying the active length by shorting the coil at the location of the contact. A problem with this type of device is the need to provide frequent maintenance of the contacts and coil to keep them clean and free of corrosion so as to provide good conductivity. Examples of this type of device are taught in U.S. Pat. No. 2,103,646 (Schlesinger), U.S. Pat. No. 2,855,599 (Kandoian), U.S. Pat. No. 2,874,274 (Adams et al.), U.S. Pat. No. 2,993,204 (Macalpine), U.S. Pat. No. 3,999,185 (Richie et al.), U.S. Pat. No. 4,117,495 (Hotchstein), U.S. Pat. No. 4,620,194 (Bel Moratalla), U.S. Pat. No. 4,958,163 (Leonard), U.S. Pat. No. 5,175,526 (Martin), and U.S. Pat. No. 5,990,841 (Sakamoto et al.). An alternative to the moveable contact is a tapped inductor, consisting of a helical wound coil with conductors attached at specific winding locations along the coil. These conductors are then connected to a multi-position switch external to the coil that is used to select the amount of inductance. This arrangement moves the maintenance requirement from the coil contact to the switch contact. Devices that commonly use tapped inductors are tuning circuits in RF linear amplifiers and common antenna tuners used to match a non-resonant antenna to a transmitter.
These contact devices are common in manually adjusted antennas also. U.S. Pat. No. 2,839,752 (Webster), U.S. Pat. No. 2,894,260 (Ellis), U.S. Pat. No. 3,653,053 (St. Vrain et al.), U.S. Pat. No. 3,798,654 (Martino et al.), U.S. Pat. No. 4,064,474 (Adams et al.), U.S. Pat. No. 4,080,604 (Wosniewski), U.S. Pat. No. 4,958,163 (Leonard), U.S. Pat. No. 6,275,195 (Gyenes) and U.S. Pat. No. 6,496,154 (Gyenes) are examples. U.S. Pat. No. 4,163,981 (Wilson) shorts the end coils of the inductor together to vary the effective length as an alternative to a contactor, however this has the same potential for corrosion as a contact.
Devices that do not use contacts may move two coils relative to each other to adjust inductance, such as U.S. Pat. No. 1,819,904 (Love) and U.S. Pat. No. 3,541,554 (Shirey). Alternately the inductance coil may be wound as the antenna is adjusted such as U.S. Pat. No. 3,226,725 (Ritchie et al.) and U.S. Pat. No. 4,139,852 (Koyanagi). Another scheme is to move a metallic or ferrite core inside the inductance coil as in U.S. Pat. No. 3,264,647 (Nuttle). The disadvantage of this is such core inductors have objectionable non-linear effects. These schemes also have complicated mechanisms and/or have undesirable electrical characteristics reducing their effectiveness.